I have a motorhome on a fiat Ducato. I removed the battery to charge it, (as it was flat) However, when I went to start up the van, there was no lights on the dash and was unable to start it. It has a Toad immobilser and I have tried everything to get it going. I can open/close the van with the Fiat key, but thats about all I can do. Any ideas?

I would start by checking the mega fuses on the battery positive terminal board. what is the battery voltage and voltages on the equipment side of the fuses?